In the proposed research, we will use a newly developed primate optogenetic tool, which enables us to create reversible micro lesions within the neocortex of awake, behaving monkeys. Using this tool, we will test and quantify the role of macaque Area V4, an intermediate stage of processing within the ventral visual processing stream. In Aim 1, we will examine how this new tool modulates the gain of neuronal responses, and will quantify the region of activation as a function of illumination level. Area V4 has a patchy organization, with some regions being selective for color, and others selective for orientation. In Aim 2, we will test the role of V4 in the perception of color and of orientation, by having monkeys make fine discriminations of color and orientation, while we inactivate patches color- or orientation-selective neurons. We will examine whether activating the neurons tuned for color impairs color judgments, and not orientation judgments, and whether activating the neurons tuned for orientation impairs orientation judgments, and not color judgments. We will also examine whether this impairment is greater when we inactivate neurons tuned for the discriminanda or, alternatively, tuned such that the steep part of the tuning curve is suited to discriminate among discriminanda. In Aim 3, we will use optogenetic micro lesions to test whether V4 plays a causal role in mediating attentional selection. The data we collect will provide the most direct evidence for or against the proposition that V4 plays a causal role in the perception of color and of orientation, and whether it plays a causal role in attentional selection. In addition, the proposed research will demonstrate the use of this new optogenetic tool, and we will make it publicly available, so that other researchers working in primates and in other species, can use this tool to test theories of perception, cognition and action.